1. Field of the Invention
This invention relates to adhesives and sealants prepared from novel anionic block copolymers of mono alkenyl arenes and conjugated dienes.
2. Background of the Art
The preparation of block copolymers of mono alkenyl arenes and conjugated dienes is well known. One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. No. 3,595,942 and U.S. Pat. No. Re. 27,145. Such polymers are broadly termed Styrenic Block Copolymers or SBC's.
SBC's have a long history of use as adhesives and sealants. For example, U.S. Pat. No. 3,239,478 (“Harlan”) discloses adhesives comprising unsaturated styrene-isoprene-styrene block copolymers (“SIS”) and styrene-butadiene-styrene block copolymers (“SBS”) in adhesives and sealants. Harlan also broadly discloses adhesives comprising the hydrogenated S-B-S (i.e. “SEBS”) and hydrogenated S-I-S (i.e. “SEPS”) block copolymers with tackifying resins and extender oils for a variety of adhesives and sealants, including pressure sensitive adhesives. It is known in the art that resins which are compatible with the rubber segment of the SBC's are responsible for development of tack in pressure sensitive adhesives. Heretofore, it has not been possible to prepare SBC's in which the compatibility characteristics of the rubber segment could be controlled by the polymer manufacturer. Therefore, much work has been done by resin manufacturers to develop new resins tailored for optimum compatibility with each of the different types of SBC. For example, aliphatic resins have good compatibility with SIS and SEBS (i.e. hydrogenated SBS polymers) type SBC's but not with SBS type. New aliphatic/aromatic resins have been developed which have good compatibility with SBS type SBC's. This invention comprises a new class of SBC's in which the polymer manufacturer can vary the compatibility characteristics of the rubber segment.
It is also known in the literature that unsaturated SBS and SIS polymers are susceptible to oxidative degradation during processing in hot melt adhesives and sealants and that the hydrogenated analogs have much better resistance to degradation during hot melt processing. The rubber segments in the hydrogenated SBC's are more highly entangled than the rubber segments in the unsaturated SBS and SIS polymers. Therefore, in formulations at a constant polymer content, PSA based on the hydrogenated SBC's generally have poorer tack than PSA based on the unhydrogenated SBC's. It would be desirable to use blends of hydrogenated and unsaturated SBC's to get the best balance of melt stability and tack. However, due to differences in compatibility, adhesives based on blends of SEBS and SIS polymers, for example, were not compatible. Usually adhesives based on these blends are hazy initially and phase separate upon standing. Adhesive films based on these blends lacked the high gloss of adhesives based on compatible adhesives and usually the properties of these adhesives were poor.
Now a novel anionic block copolymer based on mono alkenyl arene end blocks and controlled distribution mid blocks of mono alkenyl arenes and conjugated dienes has been discovered and is described in copending, commonly assigned U.S. patent application Ser. No. 60/355,210, entitled “NOVEL BLOCK COPOLYMERS AND METHOD FOR MAKING SAME”. Methods for making such polymers are described in detail in the above-mentioned patent application. Patentees have found that the resin compatibility of these new polymers is indeed different from conventional hydrogenated SBC's which will allow resin manufacturers the opportunity to develop new resins which are compatible with these novel SBC's. Patentees have also found that phase stable, clear adhesives can be made using blends of unsaturated SBC's, especially certain SIS polymers, with these new polymers. This allows formulators to develop hot melt PSA's which have the good melt stability found for hydrogenated SBC's but better tack.